1. Field of the Invention
The present invention relates to a computer controlled fluidic pressure, flow rate or position control valve, especially a control valve for use in fluid flow systems requiring more than one fluid pressure level, flow rate or load position. The control system provides a control pressure flow rate or position in either a continuously variable fashion, or by effectuating pressure flow rate or position changes in sequential, discrete steps.
2. Description of the Prior Art
Numerous mechanical and electrical systems have been devised for controlling fluid flow in industrial and manufacturing environments. Fluid systems which require precise control are employed in numerous phases of industry, including the control of robots, the operation of presses for manufacturing rubber and plastic parts and for tensioning devices in the paper industry. The invention may also be used in mobile and nonmobile applications, in nuclear reactors, on aircraft and in missiles, and in petroleum refining operations, as well as in numerous other applications. In many of the industrial applications to which the invention is applicable, pressurized fluid is typically circulated from a high pressure source, through a load, and then exhausted to a low pressure supply or reservoir. It is highly important to precisely control and alter the amount of fluid flowing in such systems in order to achieve the desired results.
One very common system for controlling and altering fluid flow is through the use of a relief valve or system of relief valves. A relief valve is found in most fluidic systems. A relief valve is a normally closed valve connected between a pressure line and a reservoir. The purpose of the relief valve is to limit pressure in the system to a preset maximum by diverting some or all of the fluid flow from the high pressure line to a bypass line leading to a tank or other type of reservoir.
The simplest form of control of a relief valve is through a manual setting. That is, the volume of fluid flow through the bypass or relief line is adjusted simply by turning a knob which increases or decreases the orifice between a valve element, such as a poppet, and a valve seat leading to a bypass line to the tank. With the increased requirement for precision in modern industrial and processing operations, manually set relief valves are totally inadequate.
Due to the difficulties in achieving immediate and precision control of relief valves, some industrial systems employ multiple preset valves. In the conventional multiple valve system, several pressure valves are utilized. Directional valves are used to direct the system to "look" at these various pressure valves. Not only are such systems expensive, due to the repetition of components, but they also require excessive ducting and room to accomodate the multiple components. The considerable weight and size of this type of installation renders it impractical for many applications. Also, the engineering cost in designing such a system is quite high since extreme care must be taken to avoid the loss of efficiency due to the complexity of ducting.
In an attempt to avoid the problems and limitations associated with the multiple valve system, servo-type pressure control valves were designed. Typically the servo-valves were flapper type valves. The servo valve did eliminate the size and weight problems of the multiple pressure valve system, but sacrificed performance in doing so.
In a typical installation, the flapper nozzle type servo valve is an electrically modulated pilot to a compound pressure valve. The flapper is analogous to a poppet and seat with a spring type of control in that it is a force balanced system. When current is applied to the coil, the resultant magnetic force pulls the flapper toward the nozzle. This results in a smaller flow orifice and a buildup of pressure in the valve. The increased pressure produces a force which opposes the magnetic force and a balance is achieved with pressure being approximately proportional to the current applied. Because the coil used must be reasonably compact, the nozzle orifice must be small. The resulting effect is a fluid passage with a small cross sectional area capable of passing only very low volumes of fluid. In application, this severely restricts the speed of operation of the system. Commercially available valves of this type have a response time of approximately 250 milliseconds.
A further problem associated with this type of valve is that because of the narrow passages required, a high degree of filtration is necessary to prevent contamination that would otherwise block the passages and cause malfunctions. Also, because of the narrow passages, changes in fluid density inordinantly change the fluid flow characteristics and result in excessive fluid pressure fluctuations.
In servo-type pressure control valves the electrical analog signal necessary to vary the current of the coil is normally generated by a power supply. Generally the power supply contains a dither to eliminate inherent fluid stiction of the valve. The dither frequency capabilities of existing systems adversely affect the response time of the analog servo valve.
Typically conventional fluid flow control systems of the type described do not provide feedback to influence the fluid pressure. Accordingly, such systems do not compensate for thermal effects, amplifier drift, changes in ambient temperature or component aging. For many applications, these conventional fluid pressure control devices have proven unsatisfactory.